Offer "quit and update" in the footer, check for updates on every start

The update check ran at most once a day and only printed a line after the
TUI exited. Now the registry is asked on every start (in the background,
3 s timeout, no cache) and the result reaches the TUI as a message.

With a newer release, the footer shows `q quit and update to vX` right of
help and quit, on a highlighted background (Theme.Highlight). `q` quits
and `main` installs the release once the terminal is restored, sharing
the code of `submarine update`; plain quit moves to `Q`, ctrl+c never
updates.

Drop LatestCached and CheckInterval. Add D27 and F58.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/tui/theme/help.go

@@ -1,6 +1,9 @@
 package theme
 
-import "github.com/charmbracelet/bubbles/help"
+import (
+	"github.com/charmbracelet/bubbles/help"
+	"github.com/charmbracelet/lipgloss"
+)
 
 // NewHelp returns a help renderer in the theme's colors.
 func NewHelp(t Theme) help.Model {
@@ -13,3 +16,9 @@ func NewHelp(t Theme) help.Model {
 	h.Styles.FullSeparator = h.Styles.ShortSeparator
 	return h
 }
+
+// Highlight is the style of a key hint that should catch the eye: dark text
+// on the accent color, on the whole hint including its key.
+func (t Theme) Highlight() lipgloss.Style {
+	return lipgloss.NewStyle().Foreground(lipgloss.Color("0")).Background(t.Primary).Bold(true)
+}